CN114374441A - Quantum key distribution phase decoding device for immune channel disturbance - Google Patents

Quantum key distribution phase decoding device for immune channel disturbance Download PDF

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CN114374441A
CN114374441A CN202210284760.4A CN202210284760A CN114374441A CN 114374441 A CN114374441 A CN 114374441A CN 202210284760 A CN202210284760 A CN 202210284760A CN 114374441 A CN114374441 A CN 114374441A
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polarization
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beam splitter
polarization beam
output port
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CN114374441B (en
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赵义博
王东
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Beijing Zhongkeguoguang Quantum Technology Co ltd
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Beijing Zhongkeguoguang Quantum Technology Co ltd
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/70—Photonic quantum communication
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/25—Arrangements specific to fibre transmission
    • H04B10/2507—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L9/00—Cryptographic mechanisms or cryptographic arrangements for secret or secure communications; Network security protocols
    • H04L9/08—Key distribution or management, e.g. generation, sharing or updating, of cryptographic keys or passwords
    • H04L9/0816—Key establishment, i.e. cryptographic processes or cryptographic protocols whereby a shared secret becomes available to two or more parties, for subsequent use
    • H04L9/0852—Quantum cryptography

Abstract

A quantum key distribution phase decoding device for immune channel disturbance comprises a first polarization beam splitter, a polarization interferometer, a circulator, a first single-photon detector and a second single-photon detector; the first port of the circulator is used as the input port of the decoding device; a third port of the circulator is connected with the first single-photon detector; a first input port and a second input port of the first polarization beam splitter are respectively connected with a second port of the circulator and the second single-photon detector; the first output port and the second output port of the first polarization beam splitter are respectively connected with the first port and the second port of the polarization interferometer through the first optical fiber and the second optical fiber. Compared with the prior art, the invention can realize immune channel disturbance without an active polarization compensation module, thereby improving the stability of the system. And the two polarization component pulses respectively pass through the primary polarization interferometer, which is equivalent to the fact that the whole pulse passes through the primary polarization interferometer, and the loss of a receiving end cannot be additionally increased.

Description

Quantum key distribution phase decoding device for immune channel disturbance
Technical Field
The invention relates to the technical field of quantum phase coding, in particular to a quantum key distribution phase decoding device for immune channel disturbance.
Background
Quantum key distribution can provide unconditionally secure key distribution for both communication parties at a long distance, and the most mature protocol at present is the BB84 quantum key distribution protocol. The optical fiber quantum key distribution system generally adopts a single-mode optical fiber as a transmission channel, but because the optical fiber channel has an inherent birefringence effect, the polarization state of photons can change in the transmission process and can change along with the change of the external environment. Phase encoding is widely used because it encodes information into a phase difference between two time modes before and after a quantum state, and is very stable when transmitted in an optical fiber channel. However, when the traditional scheme based on the double unequal arm mach-zehnder interference ring performs decoding interference at the receiving end, the polarization state is randomly changed due to the disturbance of the optical fiber channel, so that the stability of the interference is affected, and therefore, the system is poor in stability and is easily subjected to environmental interference.
If polarization tracking and compensation are performed at the receiving end through feedback control, system complexity is increased, time and resource consumption is caused, and the error rate is high. Therefore, the prior art generally adopts a mode of passively compensating the polarization state, such as Plug-and-play (Plug-and-play) round-trip type quantum key distribution system, and uses the characteristic that a faraday mirror rotates the polarization state of incident light by 90 degrees to counteract the effect of the fiber channel on the polarization state of photons, thereby ensuring the stability of the system. However, the system is vulnerable to Trojan attack, the operating frequency of the system is limited, and the Raman scattering effect of the optical fiber also increases the system noise. Another solution is to use a faraday-michelson interferometer, so that the fiber birefringence effect and the influence of environmental disturbance on the polarization state can be eliminated, and the system is very stable. But because the light pulse will pass through the phase modulator 2 times, the loss of the receiving end is increased, and the efficiency of the system is reduced.
Disclosure of Invention
Aiming at the defects in the prior art, the invention provides a quantum key distribution phase decoding device for immune channel disturbance.
The technical scheme of the invention is realized as follows:
a quantum key distribution phase decoding device for immune channel disturbance comprises a first polarization beam splitter, a polarization interferometer, a circulator, a first single-photon detector and a second single-photon detector, wherein the first polarization beam splitter comprises a first input port, a second input port, a first output port and a second output port; the polarization interferometer comprises a first port and a second port; the circulator comprises a first port, a second port and a third port; the first port of the circulator is used as an input port of a decoding device; the third port of the circulator is connected with the first single-photon detector; the first input port and the second input port of the first polarization beam splitter are respectively connected with the second port of the circulator and the second single-photon detector; the first output port and the second output port of the first polarization beam splitter are respectively connected with the first port and the second port of the polarization interferometer through a first optical fiber and a second optical fiber, and the first optical fiber and the second optical fiber are both polarization maintaining optical fibers and are respectively welded at 45 degrees.
Preferably, the polarization interferometer comprises a second polarization beam splitter comprising a first input port, a second input port, a first output port and a second output port, a first phase modulator, a first faraday mirror and a second faraday mirror; a first input port and a second input port of the second polarization beam splitter are respectively used as a first port and a second port of the polarization interferometer; a first output port of the second polarization beam splitter is connected with a first Faraday mirror through a first phase modulator to form a long arm of the polarization interferometer; and a second output port of the second polarization beam splitter is directly connected with a second Faraday mirror to form a short arm of the polarization interferometer.
Preferably, the polarization interferometer comprises a third polarization beam splitter, a fourth polarization beam splitter and a second phase modulator, the third polarization beam splitter and the fourth polarization beam splitter each comprising an input port, a first output port and a second output port; the input ports of the third polarization beam splitter and the fourth polarization beam splitter are respectively used as a first port and a second port of the polarization interferometer; the first output port of the third polarization beam splitter is connected with the first output port of the fourth polarization beam splitter through a second phase modulator to form a long arm of the polarization interferometer; and the second output port of the third polarization beam splitter is directly connected with the second output port of the fourth polarization beam splitter to form a short arm of the polarization interferometer.
Preferably, the polarization interferometer comprises a fifth polarization beam splitter comprising a first input port, a second input port, a first output port and a second output port, and a third phase modulator; a first input port and a second output port of the fifth polarization beam splitter are respectively used as a first port and a second port of the polarization interferometer; and a first output port and a second input port of the fifth polarization beam splitter are respectively connected with an input port and an output port of the third phase modulator through polarization-maintaining optical fibers to form a long arm of the polarization interferometer.
Preferably, the first and second optical fibers are equal in length.
Compared with the prior art, the invention has the following beneficial effects:
the invention can eliminate the influence of the polarization state random change on the system caused by the polarization disturbance of the channel by respectively interfering the polarization beam splitting of the phase coding state pulse and then carrying out the combination detection, can realize the immune channel disturbance without an active polarization compensation module, and improves the stability of the system. And the two polarization component pulses respectively pass through the primary polarization interferometer, which is equivalent to the fact that the whole pulse passes through the primary polarization interferometer, and the loss of a receiving end cannot be additionally increased.
Drawings
FIG. 1 is a schematic block diagram of a quantum key distribution phase decoding apparatus for immune channel perturbation according to the present invention;
FIG. 2 is a schematic block diagram of a quantum key distribution phase decoding apparatus for immune channel perturbation according to a first embodiment of the present invention;
FIG. 3 is a schematic block diagram of a quantum key distribution phase decoding apparatus for immune channel perturbation according to a second embodiment of the present invention;
fig. 4 is a schematic block diagram of a third embodiment of the quantum key distribution phase decoding apparatus for immune channel perturbation according to the present invention.
In the figure: the device comprises a first polarization beam splitter-1, a polarization interferometer-2, a second polarization beam splitter-2-1, a first phase modulator-2-2, a first Faraday mirror-2-3, a second Faraday mirror-2-4, a third polarization beam splitter-2-5, a fourth polarization beam splitter-2-6, a second phase modulator-2-7, a fifth polarization beam splitter-2-8, a third phase modulator-2-9, a circulator-3, a first single photon detector-4 and a second single photon detector-5.
Detailed Description
The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown.
As shown in fig. 1, a quantum key distribution phase decoding apparatus (hereinafter, referred to as decoding apparatus) with immune channel perturbation includes a first polarization beam splitter 1, a polarization interferometer 2, a circulator 3, a first single-photon detector 4, and a second single-photon detector 5, where the first polarization beam splitter 1 includes a first input port, a second input port, a first output port, and a second output port; the polarization interferometer 2 comprises a first port and a second port; the circulator 3 comprises a first port, a second port and a third port; a first port of the circulator 3 is used as an input port of a decoding device; a third port of the circulator 3 is connected with a first single-photon detector 4; a first input port and a second input port of the first polarization beam splitter 1 are respectively connected with a second port of the circulator 3 and the second single-photon detector 5; the first output port and the second output port of the first polarization beam splitter 1 are respectively connected with the first port and the second port of the polarization interferometer 2 through a first optical fiber and a second optical fiber. The first optical fiber and the second optical fiber are both polarization maintaining optical fibers and have equal lengths, and are respectively welded at 45 degrees, the lengths of the first optical fiber and the second optical fiber are equal, so that light entering from two directions can reach the polarization interferometer at the same time, and the phase modulation rate can be improved.
The specific decoding process is as follows:
the phase difference between the front time mode and the rear time mode of the phase coding state sent by the sending end is
Figure 922908DEST_PATH_IMAGE001
And have the same polarization, can be written as
Figure 813503DEST_PATH_IMAGE002
The polarization states of temporal modes |0> and |1> are assumed to be both horizontally polarized. After passing through a single-mode optical fiber channel, due to the existence of birefringence effect and the disturbance of the environment where the channel is located, the phase encoding state changes into a random polarization state when reaching a receiving end, and therefore the phase encoding state entering the decoding device can be written as
Figure 344979DEST_PATH_IMAGE003
Wherein the content of the first and second substances,
Figure 688235DEST_PATH_IMAGE004
。
after entering the decoding device, the phase encoding state firstly enters a first input port of a first polarization beam splitter 1 through a circulator 3, is divided into a first polarization pulse and a second polarization pulse with mutually vertical polarization, and is respectively emitted from a first output port and a second output port of the first polarization beam splitter 1, and then is subjected to 45-degree polarization rotation respectively to obtain a quantum state
Figure 330569DEST_PATH_IMAGE005
Wherein the content of the first and second substances,
Figure 75671DEST_PATH_IMAGE006
，|s>and | f>Respectively, representing propagation along the slow and fast axes of the polarization maintaining fiber.
The first polarized pulse enters the first port of the polarization interferometer 2, where the component | s, propagating along the slow axis of the polarization maintaining fiber>And the component | f propagating along the fast axis>Running the short and long arms of the polarization interferometer 2, respectively, the component of the long arm modulating the phase
Figure 512469DEST_PATH_IMAGE007
Emerging from the second port of the polarization interferometer 2 and still propagating along the slow axis and the fast axis of the polarization maintaining fiber, respectively, the quantum states can be written as
Figure 343022DEST_PATH_IMAGE008
Wherein, due to
Figure 54626DEST_PATH_IMAGE009
Time pattern of
Figure 919814DEST_PATH_IMAGE010
And component
Figure 793092DEST_PATH_IMAGE011
Time pattern of
Figure 845361DEST_PATH_IMAGE012
The optical paths are equal, they are overlapped in time, they are combined by interference when they are emitted from the second port of the polarization interferometer 2, and after 45 deg. polarization rotation they become the third polarization pulse, the quantum state can be written as
Figure 95077DEST_PATH_IMAGE013
Phase difference
Figure 814771DEST_PATH_IMAGE014
. While
Figure 858951DEST_PATH_IMAGE009
Time pattern of
Figure 398516DEST_PATH_IMAGE015
And
Figure 186344DEST_PATH_IMAGE011
time pattern of
Figure 760545DEST_PATH_IMAGE016
The non-interference peak is respectively advanced and delayed in time with respect to the third polarization pulse by T, which is the time corresponding to the arm length difference of the polarization interferometer 2. By setting the door opening time position of the single photon detector, the non-interference peak can be dropped outside the door control, and the influence of the non-interference peak on the interference peak is eliminated, so that the subsequent propagation process is not considered.
The third polarization pulse returns to the second output port of the first polarization beam splitter 1, and is transmitted along the slow axis of the polarization-maintaining fiberQuantity | s>The light beam is emitted from a first input port of the first polarization beam splitter 1 and reaches a first single photon detector 4 through a circulator 3, and the light intensity is
Figure 710046DEST_PATH_IMAGE017
Component | f propagating along the fast axis>Is emitted from a second input port of the first polarization beam splitter 1 and enters a second single photon detector 5 with the light intensity of
Figure 2487DEST_PATH_IMAGE018
。
Similarly, a second polarized pulse enters the second port of the polarization interferometer 2, where the component | s propagating along the slow axis of the polarization maintaining fiber>And the component | f propagating along the fast axis>Running the short and long arms of the polarization interferometer 2, respectively, the component of the long arm modulating the phase
Figure 328426DEST_PATH_IMAGE019
Emerging from the first port of the polarization interferometer 2 and still propagating along the slow axis and the fast axis of the polarization maintaining fiber, respectively, the quantum states can be written as
Figure 288292DEST_PATH_IMAGE020
Wherein, due to
Figure 674274DEST_PATH_IMAGE021
Time pattern of
Figure 188432DEST_PATH_IMAGE022
And component
Figure 318062DEST_PATH_IMAGE023
Time pattern of
Figure 866855DEST_PATH_IMAGE024
The optical paths are equal, they are overlapped in time, they are combined by interference when they are emitted from the first port of the polarization interferometer 2, and become the fourth polarization pulse after the polarization rotation of 45 degree, the quantum state can be written as
Figure 423738DEST_PATH_IMAGE025
Phase difference
Figure 425192DEST_PATH_IMAGE026
. While
Figure 358513DEST_PATH_IMAGE021
Time pattern of
Figure 761813DEST_PATH_IMAGE027
And
Figure 489597DEST_PATH_IMAGE023
time pattern of
Figure 978348DEST_PATH_IMAGE028
Is a non-interference peak and can be ignored.
The fourth polarized pulse returns to the first output port of the first polarization beam splitter 1, and the component | s propagating along the slow axis of the polarization maintaining fiber>The light beam is emitted from a first input port of the first polarization beam splitter 1 and reaches a first single photon detector 4 through a circulator 3, and the light intensity is
Figure 715359DEST_PATH_IMAGE029
Component | f propagating along the fast axis>Is emitted from a second input port of the first polarization beam splitter 1 and enters a second single photon detector 5 with the light intensity of
Figure 973165DEST_PATH_IMAGE030
。
Therefore, the detection result of the first single-photon detector 4 and the second single-photon detector 5 is the sum of the light intensities of the interference of the third polarized pulse and the fourth polarized pulse respectively, that is, the sum is
Figure 871851DEST_PATH_IMAGE031
And
Figure 847898DEST_PATH_IMAGE032
random perturbations of the channel can be immune regardless of the incident polarization state. It can be seen that the maximum intensity is 1/2, since there is a non-interfering peak, and only half of the pulses interfere, so the photon has an energy efficiency of 1/2. When the transmitting end modulates 4 phases, the receiving end can modulate 2 phases for decoding, and the corresponding single photon detector response normalization probability is shown in table 1
Table 1: detector response normalized probability table
Figure 123021DEST_PATH_IMAGE033
Fig. 2 shows a decoding apparatus according to a first embodiment of the present invention:
the structure of the decoding device is as follows: the polarization interferometer 2 comprises a second polarization beam splitter 2-1, a first phase modulator 2-2, a first Faraday mirror 2-3 and a second Faraday mirror 2-4, wherein the second polarization beam splitter 2-1 comprises a first input port, a second input port, a first output port and a second output port; the first input port and the second input port of the second polarization beam splitter 2-1 are respectively used as a first port and a second port of the polarization interferometer 2; a first output port of the second polarization beam splitter 2-1 is connected with a first Faraday mirror 2-3 through a first phase modulator 2-2 to form a long arm of the polarization interferometer 2; and a second output port of the second polarization beam splitter 2-1 is directly connected with a second Faraday mirror 2-4 to form a short arm of the polarization interferometer 2.
An embodiment of a decoding process includes:
the phase difference between the front time mode and the rear time mode of the phase coding state sent by the sending end is
Figure 235334DEST_PATH_IMAGE034
And have the same polarization, can be written as
Figure 570500DEST_PATH_IMAGE035
The polarization states of temporal modes |0> and |1> are assumed to be both horizontally polarized. After passing through a single-mode optical fiber channel, due to the existence of birefringence effect and the disturbance of the environment where the channel is located, the phase encoding state changes into a random polarization state when reaching a receiving end, and therefore the phase encoding state entering the decoding device can be written as
Figure 768263DEST_PATH_IMAGE036
Wherein the content of the first and second substances,
Figure 847078DEST_PATH_IMAGE037
。
after entering the decoding device, the phase encoding state firstly enters a first input port of a first polarization beam splitter 1 through a circulator 3, is divided into a first polarization pulse and a second polarization pulse with mutually vertical polarization, and is respectively emitted from a first output port and a second output port of the first polarization beam splitter 1, and then is subjected to 45-degree polarization rotation respectively to obtain a quantum state
Figure 813897DEST_PATH_IMAGE038
Wherein the content of the first and second substances,
Figure 585544DEST_PATH_IMAGE039
，|s>and | f>Respectively, representing propagation along the slow and fast axes of the polarization maintaining fiber.
The first polarized pulse enters the first input port of the second polarization beam splitter 2-1, where the component | s propagating along the slow axis of the polarization maintaining fiber>Transmitted by the second polarization beam splitter 2-1, emitted from the second output port, reached the second faraday mirror 2-4 via the short arm, reflected back to the second output port of the second polarization beam splitter 2-1, emitted from the second input port, and still propagating along the slow axis of the polarization maintaining fiber. Component | f propagating along the fast axis of the polarization maintaining fiber>Reflected by the second polarization beam splitter 2-1, emitted from the first output port, and modulated in phase by the long arm first phase modulator 2-2
Figure 270603DEST_PATH_IMAGE040
Then reaches the first Faraday mirror 2-3 to be reflected back to the second biasThe first output port of the vibration beam splitter 2-1 is emergent from the second input port and still propagates along the fast axis of the polarization maintaining fiber. Component | s>And | f>Can be respectively written as
Figure 887529DEST_PATH_IMAGE041
Wherein, due to
Figure 974434DEST_PATH_IMAGE042
Time pattern of
Figure 651403DEST_PATH_IMAGE043
And component
Figure 823758DEST_PATH_IMAGE044
Time pattern of
Figure 244375DEST_PATH_IMAGE045
The optical paths are equal, the two are overlapped in time, the two are combined in an interference mode when the light beam is emitted from the second input port of the second polarization beam splitter 2-1, the light beam becomes a third polarization pulse after being subjected to 45-degree polarization rotation, and the quantum state can be written as
Figure 185786DEST_PATH_IMAGE046
Phase difference
Figure 33656DEST_PATH_IMAGE047
. While
Figure 693308DEST_PATH_IMAGE042
Time pattern of
Figure 652037DEST_PATH_IMAGE048
And
Figure 979113DEST_PATH_IMAGE044
time pattern of
Figure 732305DEST_PATH_IMAGE049
Is a non-interference peak and can be ignored.
The third polarization pulse returns to the second output port of the first polarization beam splitter 1, and the component | s propagating along the slow axis of the polarization maintaining fiber>The light beam is emitted from a first input port of the first polarization beam splitter 1 and reaches a first single photon detector 4 through a circulator 3, and the light intensity is
Figure 879253DEST_PATH_IMAGE050
Component | f propagating along the fast axis>Is emitted from a second input port of the first polarization beam splitter 1 and enters a second single photon detector 5 with the light intensity of
Figure 641672DEST_PATH_IMAGE051
。
Similarly, a second polarized pulse enters a second input port of the second polarization beam splitter 2-1, where the component | s propagating along the slow axis of the polarization maintaining fiber>Reflected by the second polarization beam splitter 2-1, emitted from the second output port, reaches the second faraday mirror 2-4 through the short arm, reflected back to the second output port of the second polarization beam splitter 2-1, emitted from the first input port, and still transmitted along the slow axis of the polarization-maintaining fiber. Component | f propagating along the fast axis of the polarization maintaining fiber>Transmitted by the second polarization beam splitter 2-1, emitted from the first output port, and modulated in phase by the long arm and the first phase modulator 2-2
Figure 557676DEST_PATH_IMAGE052
Then reaches the first Faraday mirror 2-3, is reflected back to the first output port of the second polarization beam splitter 2-1, exits from the first input port and still propagates along the fast axis of the polarization-maintaining fiber. Component | s>And | f>Can be respectively written as
Figure 747349DEST_PATH_IMAGE053
Wherein, due to
Figure 116013DEST_PATH_IMAGE054
Time pattern of
Figure 416544DEST_PATH_IMAGE055
And component
Figure 452634DEST_PATH_IMAGE056
Time pattern of
Figure 813208DEST_PATH_IMAGE057
The optical paths are equal, the two are overlapped in time, the two are combined in an interference mode when the light beam is emitted from the first input port of the second polarization beam splitter 2-1, the light beam becomes a fourth polarization pulse after polarization rotation of 45 degrees, and the quantum state can be written as
Figure 669168DEST_PATH_IMAGE058
Phase difference
Figure 773391DEST_PATH_IMAGE059
. While
Figure 663986DEST_PATH_IMAGE054
Time pattern of
Figure 195462DEST_PATH_IMAGE060
And
Figure 538718DEST_PATH_IMAGE056
time pattern of
Figure 181052DEST_PATH_IMAGE061
Is a non-interference peak and can be ignored.
The fourth polarized pulse returns to the first output port of the first polarization beam splitter 1, and the component | s propagating along the slow axis of the polarization maintaining fiber>The light beam is emitted from a first input port of the first polarization beam splitter 1 and reaches a first single photon detector 4 through a circulator 3, and the light intensity is
Figure 926154DEST_PATH_IMAGE062
Component | f propagating along the fast axis>Is emitted from a second input port of the first polarization beam splitter 1 and enters a second single photon detector 5 with the light intensity of
Figure 894110DEST_PATH_IMAGE063
。
Therefore, the detection result of the first single-photon detector 4 and the second single-photon detector 5 is the sum of the light intensities of the interference of the third polarized pulse and the fourth polarized pulse respectively, that is, the sum is
Figure 724663DEST_PATH_IMAGE064
And
Figure 170688DEST_PATH_IMAGE065
random perturbations of the channel can be immune regardless of the incident polarization state. Stable phase decoding can be achieved according to table 1.
As shown in fig. 3, the second embodiment of the decoding apparatus of the present invention:
the structure of the decoding device is as follows: the polarization interferometer 2 comprises a third polarization beam splitter 2-5, a fourth polarization beam splitter 2-6 and a second phase modulator 2-7, wherein the third polarization beam splitter 2-5 and the fourth polarization beam splitter 2-6 respectively comprise an input port, a first output port and a second output port; the input ports of the third polarization beam splitter 2-5 and the fourth polarization beam splitter 2-6 are respectively used as a first port and a second port of the polarization interferometer 2; the first output port of the third polarization beam splitter 2-5 is connected with the first output port of the fourth polarization beam splitter 2-6 through a second phase modulator 2-7 to form a long arm of the polarization interferometer 2; and the second output port of the third polarization beam splitter 2-5 is directly connected with the second output port of the fourth polarization beam splitter 2-6 to form a short arm of the polarization interferometer 2.
The second specific decoding process includes:
the phase difference between the front time mode and the rear time mode of the phase coding state sent by the sending end is
Figure 770297DEST_PATH_IMAGE066
And have the same polarization, can be written as
Figure 643575DEST_PATH_IMAGE067
The polarization states of temporal modes |0> and |1> are assumed to be both horizontally polarized. After passing through a single-mode optical fiber channel, due to the existence of birefringence effect and the disturbance of the environment where the channel is located, the phase encoding state changes into a random polarization state when reaching a receiving end, and therefore the phase encoding state entering the decoding device can be written as
Figure 961423DEST_PATH_IMAGE068
Wherein the content of the first and second substances,
Figure 211139DEST_PATH_IMAGE069
。
after entering the decoding device, the phase encoding state firstly enters a first input port of a first polarization beam splitter 1 through a circulator 3, is divided into a first polarization pulse and a second polarization pulse with mutually vertical polarization, and is respectively emitted from a first output port and a second output port of the first polarization beam splitter 1, and then is subjected to 45-degree polarization rotation respectively to obtain a quantum state
Figure 665254DEST_PATH_IMAGE070
Wherein the content of the first and second substances,
Figure 709434DEST_PATH_IMAGE071
，|s>and | f>Respectively, representing propagation along the slow and fast axes of the polarization maintaining fiber.
The first polarized pulse enters the input port of the third polarization beam splitter 2-5, where the component | s propagating along the slow axis of the polarization maintaining fiber>Transmitted by the third polarization beam splitter 2-5, emitted from the second output port, and reaches the second output port of the fourth polarization beam splitter 2-6 through the short arm, emitted from the input port, and still propagates along the slow axis of the polarization-maintaining fiber. Component | f propagating along the fast axis of the polarization maintaining fiber>Reflected by the third polarization beam splitter 2-5, emitted from the first output port, and modulated in phase by the long arm second phase modulator 2-7
Figure 514579DEST_PATH_IMAGE072
Then reaches the first output port of the fourth polarization beam splitter 2-6, exits from the input port and still propagates along the fast axis of the polarization maintaining fiber. Component | s>And | f>Can be respectively written as
Figure 567985DEST_PATH_IMAGE073
Wherein, due to
Figure 876607DEST_PATH_IMAGE074
Time pattern of
Figure 357267DEST_PATH_IMAGE075
And component
Figure 649708DEST_PATH_IMAGE076
Time pattern of
Figure 710068DEST_PATH_IMAGE077
The optical paths are equal, the two are overlapped in time, the two are combined in an interference mode when the light beams are emitted from the input ports of the fourth polarization beam splitters 2-6, the light beams become third polarization pulses after polarization rotation of 45 degrees, and the quantum state can be written as
Figure 404354DEST_PATH_IMAGE078
Phase difference
Figure 55915DEST_PATH_IMAGE079
. While
Figure 570073DEST_PATH_IMAGE074
Time pattern of
Figure 699703DEST_PATH_IMAGE080
And
Figure 248496DEST_PATH_IMAGE076
time pattern of
Figure 805380DEST_PATH_IMAGE081
Is a non-interference peak and can be ignored.
The third polarization pulse returns to the second output port of the first polarization beam splitter 1, and the component | s propagating along the slow axis of the polarization maintaining fiber>The light beam is emitted from a first input port of the first polarization beam splitter 1 and reaches a first single photon detector 4 through a circulator 3, and the light intensity is
Figure 806834DEST_PATH_IMAGE082
Component | f propagating along the fast axis>Is emitted from a second input port of the first polarization beam splitter 1 and enters a second single photon detector 5 with the light intensity of
Figure 5734DEST_PATH_IMAGE083
。
Similarly, the second polarized pulse enters the input port of the fourth polarization beam splitter 2-6, where the component | s propagating along the slow axis of the polarization maintaining fiber>Transmitted by the fourth polarization beam splitter 2-6, emitted from the second output port, and reaches the second output port of the third polarization beam splitter 2-5 through the short arm, and emitted from the input port, and still propagates along the slow axis of the polarization-maintaining fiber. Component | f propagating along the fast axis of the polarization maintaining fiber>Reflected by the fourth polarization beam splitter 2-6, emitted from the first output port, and modulated in phase by the long arm second phase modulator 2-7
Figure 409033DEST_PATH_IMAGE084
Then reaches the first output port of the third polarization beam splitter 2-5, exits from the input port and still propagates along the fast axis of the polarization maintaining fiber. Component | s>And | f>Can be respectively written as
Figure 136818DEST_PATH_IMAGE085
Wherein, due to
Figure 625568DEST_PATH_IMAGE086
Time pattern of
Figure 85282DEST_PATH_IMAGE087
And component
Figure 343088DEST_PATH_IMAGE088
Time pattern of
Figure 507353DEST_PATH_IMAGE089
The optical paths are equal, the two are overlapped in time, the two are combined in an interference mode when the light beams are emitted from the input ports of the third polarization beam splitters 2-5, the light beams become fourth polarization pulses after polarization rotation of 45 degrees, and the quantum state can be written as
Figure 483399DEST_PATH_IMAGE090
Phase difference
Figure 492944DEST_PATH_IMAGE091
. While
Figure 870835DEST_PATH_IMAGE086
Time pattern of
Figure 940423DEST_PATH_IMAGE092
And
Figure 669344DEST_PATH_IMAGE088
time pattern of
Figure 482579DEST_PATH_IMAGE093
Is a non-interference peak and can be ignored.
The fourth polarized pulse returns to the first output port of the first polarization beam splitter 1, and the component | s propagating along the slow axis of the polarization maintaining fiber>The light beam is emitted from a first input port of the first polarization beam splitter 1 and reaches a first single photon detector 4 through a circulator 3, and the light intensity is
Figure 714978DEST_PATH_IMAGE094
Component | f propagating along the fast axis>Is emitted from a second input port of the first polarization beam splitter 1 and enters a second single photon detector 5 with the light intensity of
Figure 221045DEST_PATH_IMAGE095
。
Therefore, the detection result of the first single-photon detector 4 and the second single-photon detector 5 is the sum of the light intensities of the interference of the third polarized pulse and the fourth polarized pulse respectively, that is, the sum is
Figure 906105DEST_PATH_IMAGE096
And
Figure 523031DEST_PATH_IMAGE097
random perturbations of the channel can be immune regardless of the incident polarization state. Stable phase decoding can be achieved according to table 1.
As shown in fig. 4, a third embodiment of the decoding apparatus of the present invention:
the structure of the decoding device is as follows: the polarization interferometer 2 comprises a fifth polarization beam splitter 2-8 and a third phase modulator 2-9, the fifth polarization beam splitter 2-8 comprising a first input port, a second input port, a first output port and a second output port; a first input port and a second output port of the fifth polarization beam splitter 2-8 are respectively used as a first port and a second port of the polarization interferometer 2; and the first output port and the second input port of the fifth polarization beam splitter 2-8 are respectively connected with the input port and the output port of the third phase modulator 2-9 through polarization-maintaining optical fibers to form a long arm of the polarization interferometer 2.
The third decoding specific process of the embodiment comprises the following steps:
the phase difference between the front time mode and the rear time mode of the phase coding state sent by the sending end is
Figure 609935DEST_PATH_IMAGE098
And have the same polarization, can be written as
Figure 286904DEST_PATH_IMAGE099
The polarization states of temporal modes |0> and |1> are assumed to be both horizontally polarized. After passing through a single-mode optical fiber channel, due to the existence of birefringence effect and the disturbance of the environment where the channel is located, the phase encoding state changes into a random polarization state when reaching a receiving end, and therefore the phase encoding state entering the decoding device can be written as
Figure 459260DEST_PATH_IMAGE100
Wherein the content of the first and second substances,
Figure 614298DEST_PATH_IMAGE101
。
after entering the decoding device, the phase encoding state firstly enters a first input port of a first polarization beam splitter 1 through a circulator 3, is divided into a first polarization pulse and a second polarization pulse with mutually vertical polarization, and is respectively emitted from a first output port and a second output port of the first polarization beam splitter 1, and then is subjected to 45-degree polarization rotation respectively to obtain a quantum state
Figure 86867DEST_PATH_IMAGE102
Wherein the content of the first and second substances,
Figure 934738DEST_PATH_IMAGE103
，|s>and | f>Respectively, representing propagation along the slow and fast axes of the polarization maintaining fiber.
The first polarized pulse enters the first input port of the fifth polarization beam splitter 2-8, where the component | s propagating along the slow axis of the polarization maintaining fiber>Transmitted by the fifth polarization beam splitter 2-8, directly exits from the second output port, and still propagates along the slow axis of the polarization maintaining fiber. Component | f propagating along the fast axis of the polarization maintaining fiber>Reflected by a fifth polarization beam splitter 2-8, modulated in phase by a long arm, third phase modulator 2-9
Figure 594389DEST_PATH_IMAGE104
Then reaches the second input port, exits from the second output port and still propagates along the fast axis of the polarization maintaining fiber. Component | s>And | f>Can be respectively written as
Figure 553118DEST_PATH_IMAGE105
Wherein, due to
Figure 349035DEST_PATH_IMAGE106
Time pattern of
Figure 367807DEST_PATH_IMAGE107
And component
Figure 514755DEST_PATH_IMAGE108
Time pattern of
Figure 277174DEST_PATH_IMAGE109
The optical paths are equal, the two are overlapped in time, the two are combined in an interference mode when the light beams are emitted from the second output ports of the fifth polarization beam splitters 2-8, the light beams become third polarization pulses after polarization rotation of 45 degrees, and the quantum states can be written as
Figure 193178DEST_PATH_IMAGE110
Phase difference
Figure 117271DEST_PATH_IMAGE111
. While
Figure 751515DEST_PATH_IMAGE106
Time pattern of
Figure 52046DEST_PATH_IMAGE112
And
Figure 88135DEST_PATH_IMAGE108
time pattern of
Figure 448710DEST_PATH_IMAGE113
Is a non-interference peak and can be ignored.
The third polarization pulse returns to the second output port of the first polarization beam splitter 1, and the component | s propagating along the slow axis of the polarization maintaining fiber>The light beam is emitted from a first input port of the first polarization beam splitter 1 and reaches a first single photon detector 4 through a circulator 3, and the light intensity is
Figure 570249DEST_PATH_IMAGE114
Component | f propagating along the fast axis>Is emitted from a second input port of the first polarization beam splitter 1 and enters a second single photon detector 5 with the light intensity of
Figure 674472DEST_PATH_IMAGE115
。
Likewise, the second polarized pulse enters the second output port of the fifth polarization beam splitter 2-8, where the component | s propagating along the slow axis of the polarization maintaining fiber>Transmitted by the second polarization beam splitter 2-1, directly exits the first input port and still propagates along the slow axis of the polarization maintaining fiber. Component | f propagating along the fast axis of the polarization maintaining fiber>Reflected by a fifth polarization beam splitter 2-8, modulated in phase by a long-arm first phase modulator 2-2
Figure 299488DEST_PATH_IMAGE116
And then reaches the first output port, exits from the first input port, and still propagates along the fast axis of the polarization maintaining fiber. Component | s>And | f>Can be respectively written as
Figure 830963DEST_PATH_IMAGE117
Wherein, due to
Figure 174220DEST_PATH_IMAGE118
Time pattern of
Figure 82133DEST_PATH_IMAGE119
And component
Figure 827235DEST_PATH_IMAGE120
Time pattern of
Figure 529612DEST_PATH_IMAGE121
The optical paths are equal, the two are overlapped in time, are subjected to interference synthesis when the light beams are emitted from the first input ports of the fifth polarization beam splitters 2-8, and become fourth polarization pulses and quanta after 45-degree polarization rotationA state can be written as
Figure 625744DEST_PATH_IMAGE122
Phase difference
Figure 71769DEST_PATH_IMAGE123
. While
Figure 671378DEST_PATH_IMAGE118
Time pattern of
Figure 544656DEST_PATH_IMAGE124
And
Figure 862505DEST_PATH_IMAGE120
time pattern of
Figure 846641DEST_PATH_IMAGE125
Is a non-interference peak and can be ignored.
The fourth polarized pulse returns to the first output port of the first polarization beam splitter 1, and the component | s propagating along the slow axis of the polarization maintaining fiber>The light beam is emitted from a first input port of the first polarization beam splitter 1 and reaches a first single photon detector 4 through a circulator 3, and the light intensity is
Figure 566335DEST_PATH_IMAGE126
Component | f propagating along the fast axis>Is emitted from a second input port of the first polarization beam splitter 1 and enters a second single photon detector 5 with the light intensity of
Figure 610515DEST_PATH_IMAGE127
。
Therefore, the detection result of the first single-photon detector 4 and the second single-photon detector 5 is the sum of the light intensities of the interference of the third polarized pulse and the fourth polarized pulse respectively, that is, the sum is
Figure 415660DEST_PATH_IMAGE128
And
Figure 203487DEST_PATH_IMAGE129
random perturbations of the channel can be immune regardless of the incident polarization state. Stable phase decoding can be achieved according to table 1.
The invention also discloses a transmitting end of the quantum key distribution system, which comprises a laser, a coding device and an adjustable attenuator, wherein an input port and an output port of the coding device are respectively connected with the laser and the adjustable attenuator, the laser is used for generating optical pulses, the coding device is used for coding of various protocols and generating coded pulses, and the adjustable attenuator is used for attenuating the coded pulses to a single photon magnitude.
According to the quantum key distribution phase decoding device for immune channel disturbance, polarization beam splitting is carried out on phase coding state pulses, interference is carried out on the polarization beam splitting, then combination detection is carried out, the influence of polarization state random change on a system caused by the polarization disturbance of the channel can be eliminated, the immune channel disturbance can be realized without an active polarization compensation module, and the stability of the system is improved. And the two polarization component pulses respectively pass through the primary polarization interferometer, which is equivalent to the fact that the whole pulse passes through the primary polarization interferometer, and the loss of a receiving end cannot be additionally increased.

Claims (5)

1. The quantum key distribution phase decoding device for immune channel disturbance is characterized by comprising a first polarization beam splitter (1), a polarization interferometer (2), a circulator (3), a first single-photon detector (4) and a second single-photon detector (5), wherein the first polarization beam splitter (1) comprises a first input port, a second input port, a first output port and a second output port; the polarization interferometer (2) comprises a first port and a second port; the circulator (3) comprises a first port, a second port and a third port; a first port of the circulator (3) is used as an input port of a decoding device; a third port of the circulator (3) is connected with the first single-photon detector (4); a first input port and a second input port of the first polarization beam splitter (1) are respectively connected with a second port of the circulator (3) and the second single-photon detector (5); the first output port and the second output port of the first polarization beam splitter (1) are respectively connected with the first port and the second port of the polarization interferometer (2) through a first optical fiber and a second optical fiber, and the first optical fiber and the second optical fiber are both polarization maintaining optical fibers and are respectively welded at 45 degrees.
2. The immune channel perturbed quantum key distribution phase decoding apparatus according to claim 1, wherein said polarization interferometer (2) comprises a second polarization beam splitter (2-1), a first phase modulator (2-2), a first faraday mirror (2-3) and a second faraday mirror (2-4), said second polarization beam splitter (2-1) comprising a first input port, a second input port, a first output port and a second output port; the first input port and the second input port of the second polarization beam splitter (2-1) are respectively used as a first port and a second port of the polarization interferometer (2); a first output port of the second polarization beam splitter (2-1) is connected with a first Faraday mirror (2-3) through a first phase modulator (2-2) to form a long arm of the polarization interferometer (2); and a second output port of the second polarization beam splitter (2-1) is directly connected with a second Faraday mirror (2-4) to form a short arm of the polarization interferometer (2).
3. The immune channel perturbed quantum key distribution phase decoding apparatus according to claim 2, wherein said polarization interferometer (2) comprises a third polarization beam splitter (2-5), a fourth polarization beam splitter (2-6) and a second phase modulator (2-7), said third polarization beam splitter (2-5), said fourth polarization beam splitter (2-6) each comprising an input port, a first output port and a second output port; the input ports of the third polarization beam splitter (2-5) and the fourth polarization beam splitter (2-6) are respectively used as a first port and a second port of the polarization interferometer (2); the first output port of the third polarization beam splitter (2-5) is connected with the first output port of the fourth polarization beam splitter (2-6) through a second phase modulator (2-7) to form a long arm of the polarization interferometer (2); and the second output port of the third polarization beam splitter (2-5) is directly connected with the second output port of the fourth polarization beam splitter (2-6) to form a short arm of the polarization interferometer (2).
4. The immune channel perturbed quantum key distribution phase decoding apparatus according to claim 1, wherein said polarization interferometer (2) comprises a fifth polarization beam splitter (2-8) and a third phase modulator (2-9), said fifth polarization beam splitter (2-8) comprising a first input port, a second input port, a first output port and a second output port; the first input port and the second output port of the fifth polarization beam splitter (2-8) are respectively used as a first port and a second port of the polarization interferometer (2); and a first output port and a second input port of the fifth polarization beam splitter (2-8) are respectively connected with an input port and an output port of the third phase modulator (2-9) through polarization-maintaining optical fibers to form a long arm of the polarization interferometer (2).
5. The immune channel perturbed quantum key distribution phase decoding apparatus of any of claims 1-4, wherein the first optical fiber and the second optical fiber are equal in length.
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